This research will involve a study of the changes in chemical nature and metabolism of basic chromosomal proteins which occur at various stages in spermatogenesis in the rat. Emphasis will be placed upon a study of the role of gonadotrophic hormones and testosterone in the regulation of the synthesis and phosphorylation of the unique histone subfractions which occur in seminiferous epithelial cells of the testis. An assay procedure will be developed for the evaluation of anti-spermatogenic drugs through determination of their effects upon the ratios of histone subfractions.